Indiana enters the area and doesn't see a pathway. He takes a step (or a leap of faith) and lands on the "invisible" bridge and we, as viewers, can now see the stone bridge. He throws sand on it to make the pathway visible.
Is the bridge meant to actually be invisible from every angle (aside from the sand on it now) or is it supposed to be a sort of forced perspective illusion where it's camouflaged against the pattern of rocks in the chasm?